This invention relates to a method and a device for the formation of holes in a layer of photosensitive material.
It is notably applicable to the manufacture of micro-point cathode electron sources which are used particularly for the production of imaging devices operating by cathodoluminescence excited by field emission.
The invention allows, for example, manufacturing of flat micro-point screens of a large size, for example greater than 14 inches (about 35 cm), and even flat micro-point screens having a surface area close to 1 m.sup.2.
It is of course understood that screens of a size significantly greater can be developed thanks to this invention.
Micro-point cathode electron sources and their manufacturing methods are described, for example in documents (1), (2), (3) an (4) given as references at the end of this description to which reference can be made.
To facilitate the understanding of the technical problem which has been solved by this invention, a known example of a method of manufacturing a micro-point cathode electron source is described below.
Reference will be made to FIGS. 1 to 3 of the appended drawings.
FIG. 1 shows a structure already developed which includes a substrate 2 with an insulator 4 mounted thereon, a system of cathode conductors 6, a resistive layer 7 and superimposed intersecting grids 8, with an intermediate insulator 10, and a layer 12, for example of nickel, deposited on the surface, used as a mask during the micro-point production operations.
This layer 12 of nickel, the grids 8 and the insulator 10 have holes 14, at the bottom of which micro-points will be subsequently deposited comprising a metal conductor electrically linked to the cathode conductors 6 through the resistive layer 7.
The production of the micro-points will now be explained with reference to FIG. 2.
First one begins by carrying out, for example, the deposition of a layer of molybdenum 16 on the entire structure.
This layer 16 has a thickness of about 1.8 .mu.m.
It is deposited at an angle of incidence normal to the surface of the structure.
This deposition technique allows obtaining cones 18 of molybdenum housed in the holes 14 and having a height of 1.2 to 1.5 .mu.m.
These cones comprise the electron emitting micro-points.
Selective dissolution of the nickel layer 12 is then carried out by an electrochemical method, in such a way that the perforated grids 8, made for example of niobium, are separated as may be seen in FIG. 3, and the electron emitting micro-points 18 are made apparent.
With a few technological variations, the known method which has been described, with reference to FIGS. 1, 2 and 3, is one of those which have been applied up to the present time to produce the micro-points of micro-point cathode electron sources.
In order for the size and the positioning of the micro-points 18 to be correct, it is of course necessary to control perfectly the size of the holes made in the grids 8 and in the insulator 10.
The problem is therefore the following:
Holes having a mean diameter of, for example, 1.3 .mu.m or less, are to be made in all the surfaces which are to receive micro-points.
The methods used at present to produce these holes involve photolithographic methods using direct projection or the photorepetition of an elementary pattern reproduced on all these surfaces.
In the case of large size electron sources, greater than 14 inches (about 35 cm) for example, these methods rapidly become very restricting.
Direct projection requires the production of a large size mask 1 comprising submicron patterns. These patterns are generally made of metal deposited in a thin layer on a silica or glass substrate.
This mask is difficult to make when greater than 14 inches diagonal, with the traditional techniques used in micro-electronics.
With regard to photorepetition, a small size mask is used, the size being determined by the resolution of the patterns used.
For a resolution of 1 .mu.m, a mask with a side of 20 to 50 mm is used, which requires the insolation operation necessary for the photolithography to be repeated a large number of times in order to cover the total surface of the electron source.
Both of these methods (the one using direct projection and the other photo-repetition) are therefore difficult to apply to the production of electron sources of large size.
One aim of this invention is to provide a device and a method allowing the formation of holes in a uniform fashion on large surfaces and which does not have the limitations of the above mentioned methods.
Another aim is to provide a method and a device allowing the simultaneous production of all the holes by a single insolation step. Still another aim is to provide an implementation method which is simple, has a low cost and is suited to the requirements of mass production.
An aim of the invention is also to apply the method to the manufacture of electron sources.